Many electronic components such as LSIs, ICs, hybrid ICs, transistors, diodes, thyristors, and hybrid components thereof are usually sealed using a resin composition for electronic component sealing in order to protect an electronic circuit or package from the influence of contamination, moisture, etc. from the external environment. Such an electronic component sealing material is required to suppress failure due to ionic impurities in starting materials or moisture entering from the outside as well as to have various properties such as flame retardancy, high adhesion, anti-cracking properties, and electrical properties such as high volume resistivity.
The resin composition for electronic component sealing often employs an epoxy resin that has good adhesion to metal wiring or a semiconductor chip and high heat resistance, and the resin composition for sealing often comprises, in addition to an epoxy compound as a main component, an epoxy compound curing agent, a curing promoter, an inorganic filler, a flame retardant, a pigment, a silane coupling agent, etc.
It is conventionally known that, although an epoxy resin is an excellent resin for sealing, it contains trace amounts of ionic impurities, and it is also known that there is a possibility that ionic impurities will corrode metal wiring of aluminum, etc. used in a wiring circuit for a semiconductor chip. On the other hand, accompanying recent increases in integration and increases in the speed of semiconductor circuits, there has been a great increase in the temperature of semiconductor chips due to Joule heating generated when the circuit is operating; sealing materials comprise large amounts of flame retardants such as antimony oxide, brominated epoxy resins, inorganic hydroxides, etc., and because of these flame retardant components corrosion of metal wiring such as aluminum wiring occurs more readily. This corrosion is accelerated mainly by infiltration of moisture into the epoxy resin used as a sealing material.
In order to prevent this corrosion, an epoxy resin composition for semiconductor sealing formed by mixing an epoxy resin with a bismuth compound, which is an inorganic anion exchanger, is known. Patent Document 1 discloses a bismuth compound represented by Formula (3) below as an inorganic anion exchanger and discloses that it can be used for adsorptive fixation of impurity ions in solid materials related to the electrical/electronic field. With regard to nitrate ion (NO3)6−x in the composition it defines the necessity for 3.5≦x≦5.5; when x is greater than 5.5 due to there being few NO3 groups in the compound the ion exchange capacity or exchange rate at around neutral becomes small, and as a result of the structure becoming close to that of normal hydrated bismuth oxide, the water resistance easily deteriorates, which is a defect of normal hydrated bismuth oxide.Bi6O6(OH)x(NO3)6−x·nH2O  (3)
In Formula (3), when x is greater than 5.5, if n is 0 or 1 this corresponds to a case in which (NO3) contained in an inorganic anion exchanger is less than 2%. That is, it is known that in an inorganic anion exchanger comprising a bismuth compound, one containing (NO3) at greater than 2% has a large ion exchange capacity or exchange rate at around neutral, and the water resistance is excellent, whereas one containing less (NO3) tends to have poor water resistance.
Furthermore, as a conventional process for producing a bismuth compound, Patent Document 1 discloses a method in which an equivalent amount of alkali is added over about 2 hours to bismuth nitrate having excess nitrate and a method in which in order to make the value of the subscript x in Formula (3) larger by 1, about 1 mole of alkali is further added, and it is also disclosed that a preferred reaction temperature is in the range of 20° C. to 50° C.
Moreover, Patent Document 2 discloses a bismuth compound represented by Formula (4) below and discloses that it is excellent as an inorganic ion exchanger for removing chloride ions, and since with regard to nitrate ion (NO3)4−2x in the composition it defines the necessity for −0.18≦x≦0.29, when NO3 is converted to a proportion by mass it corresponds to 8.4% to 10.5%, and a high nitrate ion content is essential.Bi10O13+x(NO3)4−2x  (4)
Furthermore, Patent Document 2 discloses that by thermal decomposition of a bismuth compound with a ratio of (NO3) to Bi of preferably greater than 4:10 as a starting material, a crystalline bismuth compound represented by Formula (4) above can be obtained, and it states that when thermal decomposition is carried out up to 630° C. crystalline Bi2O3 is obtained.
As a production process therefor, it is known that such a bismuth compound can be obtained by hydrolyzing bismuth nitrate by reacting with an alkali solution, but it is also known that this method only gives columnar crystals having an average particle size of 5 to 10 μm (see Patent Document 3).
Patent Document 3 discloses a method in which an equimolar or greater amount of a monocarboxylic acid is added to an aqueous solution containing trivalent bismuth ion and having a pH of no greater than 1.0 to thus form a bismuth-monocarboxylic acid complex, and an alkali is subsequently added thereto to thus precipitate the complex, followed by calcination, thereby giving crystalline bismuth oxide (III) (Bi2O3) microparticles. It is disclosed here that when a monocarboxylic acid is added, the complex is precipitated at a pH of 1.8 to 5.3, and it is disclosed that when no monocarboxylic acid is added, bismuth hydroxide or bismuth oxide hydrate is deposited in the strongly acidic region of a pH of 1.0 or below; it is disclosed that the bismuth-monocarboxylic acid complex becomes spherical microparticles of crystalline bismuth oxide at 340° C., and bismuth hydroxide or bismuth oxide hydrate becomes rod-shaped bismuth oxide crystals having a nonuniform particle size when heated to 550° C.